In recent years, multiple input multiple output (MIMO) technology is primarily considered as one of technologies used in a next-generation mobile communication and wireless transmission system. The multiple input multiple output (MIMO) technology is a method for enhancing a spectral efficiency using multiple antennas to enhance a low spectral efficiency that has been a problem of the single input single output (SISO) method in the prior art.
The multiple input multiple output (MIMO) technology can be divided into a spatial multiplexing technique and a spatial diversity technique according to whether or not to transmit the same data.
The spatial multiplexing technique is a method for transmitting different data through several transmitting and receiving antennas at the same time. In other words, different data is transmitted through each transmitting antenna at the transmitter side, and suitable interference removal and signal processing is carried out for the received signals at the receiver side to obtain the transmitted data, thereby enhancing data transmission rate in proportion to the number of transmitting antennas.
The spatial diversity technique is a method for transmitting the same data through several transmitting antennas to obtain transmission diversity. In other words, the spatial diversity technique is a kind of space-time channel coding.
In such a spatial diversity technique, the same data is transmitted from several transmitting antennas, thereby greatly enhancing a transmission diversity gain. However, the spatial diversity technique is not a method for enhancing a transmission rate of data, but for enhancing the reliability of data transmission by enhancing a diversity gain.
Furthermore, the multiple input multiple output (MIMO) technology can be divided into an open loop method and a closed loop method according to whether information on channels is fed back from the receiver side to the transmitter side. Of them, the multiple input multiple output (MIMO) technology with a closed loop method may enhance a transmission rate of data using channel information fed back from the receiver side. For example, in a wireless communication system using the multiple input multiple output (MIMO) with a closed loop method, the base station at the transmitter side receives a feedback on information of transmission channels from the terminal at the receiver side, and transmits data using this.
On the other hand, a coordinated multi-point (CoMP) transmission method has been developed to reduce inter-cell interference in a multi-cell environment. Using the coordinated multi-point (CoMP) transmission method, the terminal may receive data in common from multi-cell base stations.
Furthermore, each base station may support one or more terminals at the same time using the same frequency resource to enhance the performance of the system. It may be possible to enhance the communication performance of a terminal located at an edge of the cell using the CoMP method in a multi-cell environment.
The coordinated multi-point (CoMP) transmission method may include a joint processing scheme in the form of a collaborative multiple input multiple output (MIMO) through data sharing and a coordinated scheduling/beamforming scheme for reducing inter-cell interference such as worst companion and best companion.
Of them, in order to smoothly perform the collaborative multiple input multiple output (MIMO) method such as joint processing, synchronization between multiple cells performing this should be properly carried out. However, a method for precisely synchronizing multiple cell base stations performing a collaborative multiple input multiple output (MIMO) operation such as joint processing has not be established so far.